1. Field of the Invention
The present invention relates to electromagnetic couplers suitable for use in wireless communication systems to transfer information between information communication devices disposed at a short distance from each other via an electrostatic field or an induced electric field. The present invention also relates to information communication devices equipped with the electromagnetic couplers.
2. Description of Related Art
Conventional electromagnetic couplers include one disclosed in JP-B 4345851. This electromagnetic coupler (high-frequency coupler) is formed by connecting a plate-like electrode to a series inductor and a parallel inductor via a high-frequency transmission line. Such an electromagnetic coupler is to be disposed in an information communication device, such as a transmitter and a receiver. In the cases where a transmitter and a receiver are disposed so that the electrodes of their electromagnetic couplers face each other, when the distance between the two electrodes is 2λ/15 or smaller of the wavelength λ of the frequency used, the two electrodes are coupled by the electrostatic field component of longitudinal waves to behave as one capacitance and like a band pass filter as a whole, making it possible to efficiently communicate information between the two electromagnetic couplers. Also, when the distance between the two electrodes is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, information can be transferred by using an induced electric field of longitudinal waves.
Meanwhile, when the distance between the electromagnetic couplers is greater than a certain value, information cannot be transferred. As a result, other wireless devices do not suffer interference from electromagnetic waves generated from the electromagnetic couplers, and a wireless communication system in which information communication devices equipped with the electromagnetic couplers are used does not suffer interference from other wireless communication systems. Because of these characteristics, wireless communication systems in which the conventional electromagnetic coupler is used make it possible, by using an electrostatic field or an induced electric field of longitudinal waves over a short distance, to communicate a large volume of data between information communication devices by using the UWB (Ultra Wide Band) communication system, in which wide band signals are used.
As described above, when the distance between the two electrodes is 2λ/15 or smaller of the wavelength λ of the frequency used, information can be efficiently communicated between the electromagnetic couplers by forming a band pass filter. In other words, the electrodes of the two conventional electromagnetic couplers are coupled by the electrostatic field component of longitudinal waves to behave as one capacitance, and a band pass filter is formed by the series and parallel inductors. However, when the match between the two electromagnetic couplers is not good, signal transmission efficiency is degraded.
On the other hand, in the case of wireless communications carried out by using devices provided with this electromagnetic coupler, for example, a cover of each device including a dielectric exists between the electromagnetic couplers, resulting in variations in the dielectric constant between the electromagnetic couplers. Then, variations occur in the value of the capacitance between the electrodes of the two electromagnetic couplers and in the frequency characteristics of the band pass filter, which in some cases may degrade the information transmission characteristics in the frequency band of interest. Even if the expected variations in the dielectric constant in some cases are taken into account in designing the electromagnetic couplers, in the case of wireless communications carried out by using other devices made of different materials and/or differently designed, the value of the dielectric constant between the electromagnetic couplers varies, which similarly degrades the information transmission characteristics in the frequency band of interest.
Also, when the distance between the electrodes of the two electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, information is communicated by using the induced electric field component of longitudinal waves. In this case, when the relative position of the two electromagnetic couplers and the environment are kept constant, the information transmission characteristics depend on matching conditions between the electromagnetic couplers and the feeding system. In other words, the signal intensity from the electromagnetic couplers to the communication module including the feeding system increases under a good matching condition, while the signal intensity from the electromagnetic couplers to the communication module including the feeding system decreases under a poor matching condition.
In the conventional art, electromagnetic couplers are designed so that a band pass filter is formed when the distance between the electromagnetic couplers is 2λ/15 or smaller of the wavelength λ of the frequency used; however, the matching condition when the distance between the electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used is not particularly taken into account in design. Therefore, in the case of an insufficient signal intensity when the distance between the electromagnetic couplers is in the range from 2λ/15 to 8λ/15 of the wavelength λ of the frequency used, for example, a redesign is required with a view to forming a band pass filter when the distance between the electromagnetic couplers is 2λ/15 or smaller of the wavelength λ of the frequency used. This means that much time and effort is required in designing the electromagnetic couplers. In addition, when the frequency band to be used is broad, it is required to obtain a large number of frequencies in which the matching condition is suitable, which means that even more time and effort is required.